The present disclosure relates generally to permanent magnets, and more particularly, to an apparatus and a method for diagnosing permanent magnet demagnetization of a permanent magnet synchronous motor, and an apparatus for driving a permanent magnet synchronous motor.
Permanent magnet synchronous machines (PMSM) have recently become the preferred choice for a variety of applications due to their high power density, high efficiency, and ease of control. The permanent magnet rotor is one of the most critical components in a PMSM that determines the performance, efficiency, and reliability of the motor drive system. Local or uniform demagnetization of the PM can occur due to a combination of extreme thermal, electrical, mechanical, and environmental operating stresses and results in irreversible degradation of motor performance and efficiency. When the operating point of a permanent magnet, which operates at the intersection between a demagnetization curve of the PM and a load line, is driven below the knee of the curve due to demagnetizing magnetomotive force or under normal conditions with change in operating temperature, the operating point does not retrace the demagnetization curve and according irreversible demagnetization occurs. In addition, rare-earth magnet materials are susceptible to metallurgical structure changes due to corrosion or oxidation, and this also results in demagnetization. Other possible causes of demagnetization include gradual decrease in magnet strength (domain relaxation) and damage (chipping, cracking, etc. . . . ) due to vibration, shock, or mechanical forces during operation.
If the magnet is exposed to an elevated temperature for a long period of time, it can be demagnetized permanently due to changes in the metallurgical structure, which may impair its ability to be re-magnetized. If the temperature of the magnet exceeds the Curie temperature of the material, the magnetization is reduced to zero (the material can be re-magnetized, if the metallurgical structure has not been altered).
The metallurgical/structural changes of permanent magnet may arise due to the corrosion or oxidation of the surfaces of the permanent magnet as well as a high temperature. Particularly, an NdFeB magnet is vulnerable to corrosion or oxidation due to its material characteristics. The oxidized portion of the permanent magnet with structural change grows with time, especially at elevated temperature and/or under humid or chloride containing environments. The portion with structural change is more easily demagnetized (lower flux density and coercivity), and is more brittle and may lead to total disintegration in extreme cases. As the coercivity of the permanent magnet is reduced, the magnetic flux of the permanent magnet decreases, and consequently the magnetic force becomes weaker.
Moreover, when a reverse-direction magnetic field caused by a fault current exceeds the coercivity (Hc) of a permanent magnet, the permanent magnet may be demagnetized. As an example, when a motor is run, a high fault current flows in a winding due to, for example, a short circuit switch of an inverter or a short that is caused by a breakdown in the insulation between the turns of a motor winding. This high fault current causes the demagnetization of the permanent magnet included in the motor. Furthermore, a shift in the level of a load applied to a permanent magnet or a shift in the operating point of the permanent magnet caused by temperature can have an adverse affect on the permanent magnet. For example, when the degree of change of the operating point of the permanent magnet falls outside of a certain range, a loss of magnetic energy is incurred. The loss of the magnetic energy is maintained as-is even when the load or the temperature returns to its initial state and thus permanent demagnetization occurs.
In this way, permanent magnets may be demagnetized or damaged by the complex effects of various causes, and such cases are typically monitored while motors are actually being run. When a permanent magnet is demagnetized, the torque and efficiency of a motor decrease, and performance is greatly deteriorated. Since this leads to failure of the motor and may thereby exert an adverse influence on the entire motor driving system, diagnosing the condition of the permanent magnets is essential for the reliability, efficiency and stability of the system.
One method for directly diagnosing the condition of a permanent magnet is to monitor the magnetic flux distribution of the magnet. This method requires disassembly of the motor which in turn incurs the opportunity cost of time because the motor must be stopped for a long period of time and disassembled. An additional drawback of such a method is the high-cost of measurement equipment and assembly/disassembly. In view of these constraints, a method for indirectly analyzing the condition of a magnet may be considered more preferable than a method requiring disassemble of the motor in the diagnosis of permanent magnets.
One method of indirectly analyzing the condition of a magnet is a method for indirectly measuring magnetic flux through the winding of a stator. If a voltage induced in a winding is measured when a motor rotates at a constant speed, since the voltage is induced in proportion to magnetic flux, the magnetic flux of a magnet can be measured. However, such a method requires that the load be disconnected from the motor, another motor be connected to the motor to be diagnosed, and the connected motor be rotated at a constant speed. Consequently, the method has difficulties. Furthermore, there is a method that estimates the magnetic flux of a motor based on an equivalent model of the motor while the motor is being run. However, this method requires accurate knowledge of the resistance and inductance of the motor. In addition, high-cost equipment is required for measuring such parameters, and moreover, it is difficult to obtain accurate and consistent results because the parameters vary according to temperature and saturation of the core.